1080: Visual Field

[Click comic to enlarge]Title text: I recently learned something that solved a mystery that had bugged me since childhood--why, when I looked at an analog clock, the hand would sometimes seem to take a couple seconds to start ticking. Google "stopped clock illusion".

A larger version of this image can be found by clicking the image at xkcd.com - the comic's page can also be accessed by clicking on the comic number above.

This comic shows a number of vision related facts, arranged in a way that they all fit inside your field of vision (the conic area in which you can see at any given time). You're supposed to look at the center of the image while standing about a foot away from the screen (although obviously you can't read the text on the image while staring at the center).

Firstly, there's detail. The eye always sees objects closer to the eye with more detail, which Randall illustrates with progressively smaller images, which are seen with the same level of detail (remember that you're supposed to be looking at the center of the image). This is because the retina is denser near the fovea, in the center.

Next, there's the topic of night vision. The colour-seeing cone cells don't work so well in the dark, whereas the black-and-white-seeing rod cells do. The rod cells can see shapes well, whereas the cone cells see detail (such as change in colour), which Randall uses to explain why we can't read at night.

Polarization direction can be visible when quickly changing your viewing angle. Polarization is essentially the vertical direction of waves. Light, being a wave, has a direction, and is thus polarized. Polarized lenses, for example, would have "slits" to allow only light that is polarized in a certain direction to come through (blocking the light in other directions). LCD screens operate on the principle of blocking and rotating polarized light.

Floaters are deposits within the eye's vitreous humour. While normally transparent, they can occasionally cause refraction of light, making them visible, particularly on bright, blue surfaces. Randall points out that while some floaters are caused by breakdown over time, the others have a more mysterious origin.

Randall also points out that colours are mostly seen near the center of our vision, with our brain keeping track of the colours of things near the outside of our visual field. The cones of blue, red and green in the third quadrant also show how red and green's sensitivity is mostly limited to the center of our vision, whereas we can see blue in a larger field of vision. Our ability to perceive saturation (the intensity of colours) is also stronger near the center of our vision.

The left and right blind spot are the locations of the optic disc, where there are no sensitive rod or cone cells, making a literal "blind" spot. The mention of the "T-Boz blind spot" and "Chilli blind spot" are a reference to the R&B band TLC, whose members go by the aliases "Left eye", "T-Boz", and "Chilli".

An image of the moon and a supermoon also appear in the image. A supermoon is when the moon is at its closest approach to Earth and coincides with a full moon or new moon, causing it to appear larger than normal. At the sizes Randall has drawn the two moons, the difference in size (approximately ten percent) is nigh-imperceptible to the naked eye; Randall seems to be making a comment about how supermoons aren't impressive to him. That he feels like this was already indicated in panel 25 of 1052: Every Major's Terrible and then later confirmed when he published 1394: Superm*n. Here is a list of all comics referring to the term.

The "stopped clock illusion" referenced by the image text is an example of chronostasis, which is an illusion where viewing movement after changing your vision is perceived as taking a longer period of time. So when we look at a clock (which we weren't previously looking at), our field of vision has rapidly changed. The second hand on the clock thus seems to take a longer period of time to move.

[This comic contains numerous visual elements arranged around a central point, and are intended to represent locations in a sphere with the eyeball as the center. Underlaid below all of the elements are concentric circles representing degrees from straight ahead, using the eyeball's point of view, denoting where these elements would appear in someone's field of vision given proper setup. For this description, elements will be described using this grid plus location in degrees within the specified circle, placing 0 degrees to the right and going counterclockwise, separated with the word "mark".]

[At the top are the instructions to view this page]

Look at the center with your eyes this far from the screen.

[A rolled-up sheet of paper that equals about 55 total horizontal degrees in width in the measurement of the chart.]

(You can roll up a sheet of paper and cut it - or zoom the page - so it matches this image)

17 mark 0: right eye blind spot.

from 0 to 30 mark 15:

[The same image, increasing in absolute size from a very tiny object in the center to one about 20x original size at 30 degrees.]

Detail - We only see at high resolution over a small area in the center of our vision where retinal cells are densest (the fovea). If you stare at the center of this chart, your eyes are seeing all these panels at roughly the same level of detail.

9 mark 105: Moon.

7 mark 112: Supermoon.

from 0 to 20 mark 170:

[Sets 3 partially overlapping circles in multiple locations along this path. Each set has a primary color in each circle and additive colors in the overlap areas, with color saturation decreasing sharply as the sets leave the center.]

Color Vision: We don't see much color outside the center of our vision - our brains keep track of what color things are and fill it in for us.

17 mark 180: Left Eye Blind Spot.

(not pictured: T-Boz blind spot, Chilli blind spot)

From 0 to infinity mark from 180 to 205:

[A swath of blue, with heavier saturation up to 5 degrees from center to fading, but never gone out to the edges of the image.]

From 0 to 7 from 205 to 235:

[A swath of red, with full saturation in the center and fading out completely at 7 degrees from center.]

From 0 to 7 from 235 to 270:

[A swath of green, with full saturation in the center and fading out completely at 7 degrees from center.]

Red and green-sensitive cones are mainly limited to the center of our vision. We have few blue-sensitive cone cells, but they're found out to the edge of our vision.

25 mark 205: [A small whisp of white in a swath of blue.]

Blue-sky sprites: These tiny, darting spots, visible against smooth blue backgrounds, are white cells moving in the blood vessels over the retina

5 mark 195:

[A long blob, slightly distorting the blue swath.]

Floaters: Some types of floaters are caused by breakdown of your eyeball goop as you age, but this type is some other kind of debris near the retina. I don't know what.

[The same image, increasing in absolute size from a very tiny object in the center to one about 20x original size at 30 degrees. The brightness of the image varies from black at 2 mark 340, to gray at 5 mark 340, to nearly white at 10 mark 340, to slightly grayer at 20 mark 340, to medium gray at 30 mark 340.]

Night Vision: Cone cells (sharp, central color vision) don't work in low light, but rod cells (monochrome, low-res, non-central) do. This is why you can walk around in dim light, but not read. It's also why you can spot fainter stars by looking next to them.

Tools

It seems you are using noscript, which is stopping our project wonderful ads from working. Explain xkcd uses ads to pay for bandwidth, and we manually approve all our advertisers, and our ads are restricted to unobtrusive images and slow animated GIFs. If you found this site helpful, please consider whitelisting us.